You can really add any type of vegetable to a vegetable stock, but it's a good idea to start with the below basics, all roughly chopped or halved.
Two quartered onions
2 stalks celery
1 tsp peppercorns
Put all ingredients in a large stock pot and cover with about 1 inch of water.
Bring the water to a boil and then back down to a simmer. You only want to see see one or two bubbles coming up at a time.
Cook gently for about 1 ½ hours uncovered, or until vegetables are soft. Taste and keep cooking if you want the flavour more concentrated.
Strain, pressing on vegetables to remove all juices. Season to taste.
Stock will keep, refrigerated, about four days or freeze for up to four months.